An entire industry has developed around the use of dried test elements for blood analyzers that contain the necessary reagents "all in the slide". Because of the high precision capable from such test elements, it is essential that patient samples be dispensed both with correspondingly high volumetric precision and consistent wetted area. More specifically, in the dispensing of about 10 .mu.l volumes, the precision needs to be within 1% or less of the nominal value. This is not a trivial feat, since patient blood sera have viscosities that can vary from 1 to 20 cps, and a surface tension that can vary from 35 to 72 mN/m. What makes the task even more difficult is the fact that, for each assay to be run on the varying test elements, a different surface wettability is often presented to the dispensing station. Any chemistries encouraging non-wetting cause the dispensed liquid to try to perfuse up the side of the already-wetted nozzle. Perfusion, of course, causes gross variations in dispensing precision. Perfusion, to the extent it occurs, can be detected in peak pressures generated in the container during dispensing.
The above bad situation is made worse by the fact that the most economical method of getting the patient sample INTO the dispensing container, is by aspiration from a gross sample supply. To avoid having to wipe the exterior of the dispensing container used to dip and aspirate, the dispensing container must be designed keeping in mind that some residual patient sample will remain on the outside surface of the dispensing container, where it can easily interfere with dispensing if it has access to the dispensing orifice. That is, at best only a small amount of residuals from the exterior surface is needed to combine with the desired amount dispensed from the interior, before the imprecision in dispensing 10 .mu.l exceeds 1%. At worst, large amounts of residuals can spontaneously fall off, contaminating equipment, test elements, or both.
The amount and location of those residuals becomes a factor of many conditions that are not always easily controlled, including the nature and concentration of sample proteins, speed of withdrawal of the dispensing container from the gross sample supply, the viscosity of this particular sample, the depth of submersion for aspiration, and the surface area of the pipette. Of these, only the last-named factor is determinative ab initio (by the container used in the analyzer), and this factor is not easily altered from specimen to specimen to meet changing needs.
The disposable dispensing container described in U.S. Pat. No. 4,347,875 goes a long way towards solving such dispensing problems. However, even it has trouble meeting universal needs, that is, those peculiar to some of the more esoteric test element chemistries, including total protein and CO.sub.2, or to peculiar patient sample conditions, e.g., IgG multiple myeloma. Therefore, dispensing with the container of the U.S. Pat. No. 4,347,875 patent can produce an occasional unsatisfactory result, manifesting itself either as volume imprecision, or in the case of liquid perfusion a failure to dispense altogether. More specifically, a nominal 10 .mu.l drop varies (in 10 dispensing events with Dade.TM. Moni-Trol.TM. ES level II general multipurpose control serum prepared with human blood and supplied ready to use with a bicarbonate diluent by American Scientific Products as a test liquid) from 9.259 .mu.l mean value (.+-.0.368) to as much as 10.583 .mu.l mean value, .+-.0.166. Better results than this are desired, for example, results in which the mean value for 10 drops is never less than 9.93 .mu.l nor more than 10.05, .+-.0.1.